At commercial and sport fish farms, interference from marine mammals such as seals and sea lions is a serious problem. The mammals prey on the fish kept in submerged fish pens, resulting in loss to the fish farms. Therefore, it is important to the fish farms to keep the mammals away from their fish pens.
Various methods of repelling marine mammals from fish farms have been employed with only minimal successes. A chemical "taste aversion" method has been used in which lithium chloride is injected into some of the fish. A mammal taking a dosed fish will become temporarily ill. Thus, a type of negative conditioning is used to keep the mammals away from the fish pens. Drawbacks to this approach are that it is not humane, and it can be hazardous if fish dosed with lithium chloride are inadvertently consumed by humans.
Most marine mammals have excellent hearing and the water in which they live is an efficient medium for transmitting sound. The transmission of underwater sounds as warnings or irritants has been considered a promising method of repelling marine mammals. One such acoustic deterrent method is introducing sounds of predators such as killer whales and gray whales into the water near the fish farms. This method has been shown to work for a while, but the animals soon learn that no predators are in the area and come back to the fish farms.
Explosives have also been used as an acoustic deterrent. Seal bombs are underwater explosives which produce a flash of light and a high amplitude sound. Cracker shells are airborne explosives which are shot from a shot gun over the heads of mammals in the water. Both explosive methods have been used with some degree of success. However, they are dangerous to handle and are also labor intensive in that a person must repeatedly interrupt his other duties at the fish farm or must be completely dedicated to the task of discharging the explosives. Also, the explosives emit acoustic signals at low frequency and are therefore not well matched to the sensitive portions of the mammals' hearing. The explosive devices can also cause loss of hearing in the mammals which is inhumane and also renders acoustic deterrence ineffective against the deaf mammals.
High frequency acoustic signals have also been used in acoustic deterrent systems as a deterrent to seals and sea lions. In these systems, the irritation works for a while to keep the mammals away from the fish pens. However, after some time, the mammals become conditioned to the sound. The desire for the fish overrides the irritation caused by the acoustic signals, and the mammals return to the pens to feed. In fact, it is believed that after prolonged use of these systems, the signals actually act to signify the presence of food and therefore attract the seals and sea lions to the fish pens.
U.S. Pat. No. 4,922,468 to Menezes describes a method and apparatus for controlling aquatic population in defined areas. In the Menezes system, various stimuli including acoustic, light, radio and magnetic are introduced into a defined region of water depending upon the particular species of animal to be affected and that species' predicted reaction to the selected stimulus. The system uses a feedback monitor and computer to monitor the species' response to the stimuli and to adapt the stimuli to changing species and conditions.